1. Field of the Invention
This invention relates to the field of voltage sources--specifically, voltage reference sources. More particularly, the invention relates to the accurate sensing of a ground node to which a voltage reference source is connected. The invention provides a voltage reference source with accurate ground sensing and a variety of pin-programmable output voltages.
2. Discussion of the Prior Art
There are many situations in which an accurate, stable voltage must be applied, as a reference, to a load which sits between two nodes in a circuit or system. A voltage source used for this purpose is referred to as a "voltage reference", "voltage reference source", "reference voltage source" or "reference source". The reference voltage or voltages supplied by such a voltage reference appear at so-called "force" output terminals. Sometimes, a reference source is provided with one or more "sense" terminals, also. A sense terminal may be connected to some node in the circuitry associated with the load, to supply to the reference source feedback information about the effect the reference voltage is producing. The reference source may use the signal applied to the sense terminal to adjust a drive point so as to maintain the force output at the desired level. In some cases, the force and sense terminals may be connected to the same node, but they may also be connected to different nodes in the circuit.
Frequently, one of the nodes to which the voltage reference is connected is a common, or ground, node in the "external" circuit or system. The circuit design for such a system frequently is based on an assumption that every element connected to such a ground node (including any reference voltage sources) rests at the same potential. However, that idealization is not physically achievable. The ground terminals of any two elements connected to the common node often will generally be at slightly different potentials. Because the potential difference is normally quite small (such as a few millivolts or less), though, the assumption that the terminals are at the same potential is usually an acceptable one. Sometimes, however, even a small potential difference is important. For example, where a high gain element in a circuit is sensitive to a voltage at a specific point, a substantial error may be induced when the ground point for the gain element and for the control voltage are at different potentials.
The physical reality of circuit construction must therefore be distinguished from the idealized representation embodied in a schematic circuit diagram. Considering some arbitrary point in the common node as a reference location, every element attached to that node is connected to the reference location through a conductor of finite resistance. Unless there is zero current in the conductor, there is a small, but finite voltage developed on the conductor by the finite current therein. If a reference voltage source is one of the elements connected to the common node, the actual voltage applied to the circuit by the reference source will not be exactly equal to the reference voltage generated by the source but, rather, will vary with the voltage developed between the source's ground terminal and the reference ground point for the system.
In recognition of this problem, voltage references have sometimes been provided with circuitry for forcing their internal ground nodes to the same potential as the "external" system's ground reference point. However, the ability of the reference source to sense any voltage error between the common node of the system and its own internal ground node is also affected by the currents in the connection between the internal ground node of the reference source and the reference point.
Indeed, in some designs, the effect of the ground-error sensing arrangement is to induce in the output of the reference source an error voltage equal to two or more times the "grounding offset voltage"--i.e., the potential difference between the internal ground node of the reference source and the reference point in the load circuit.
It is therefore an object of the present invention to provide a voltage reference source with improved sensing of a ground node.
It is a further object of the invention to provide a voltage reference source capable of providing forced and sensed voltages referenced to that ground node.
Still another object of the invention is to provide a voltage reference source which can provide both positive and negative voltages referred to a ground node.
Yet another object of the invention is to provide such a voltage reference source which is pin-programmable to provide a variety of selectable forced voltages referenced to a ground node.
Yet another object of the invention is to provide a voltage reference source in which the sensitivity to grounding offsets is minimized.